The spectrum of triply ionized sodium, Na IV, has been studied in the 2500-1300 and 415-135 Å ranges and its analysis extended. About 55 lines in the 2160-1450 Å range have been observed and classified as 3<i>s</i>-3<i>p</i> and 3<i>p</i>-3<i>d</i> combinations, and some 25 lines are either newly observed or newly resolved in the 210-135 Å range. New levels that have been established include 18 levels belonging to the 3<i>p</i> configuration, 16 levels belonging to six new 3d terms, the <sup>5</sup><i>S</i><sub>2</sub> level of 3s, and two (<sup>2</sup><i>P</i>)<sup>3</sup><i>P</i> levels of 4<i>s</i>. For seven previously known 3<i>s</i>, 3<i>d</i>, 4<i>s</i>, and 4<i>d</i> triplet terms the fine structure has been determined: 19 out of 21 possible levels have now been established. No intercombinations between the 3<i>s</i>, 3<i>p</i>, and 3<i>d</i> quintet system and the singlet-triplet system have been observed. Isoelectronic comparisons in the OI, FII, NeIII, NaIV, MgV sequence have been made as have least squares fits of theoretical energy expressions to observed 3<i>s</i>, 3<i>p</i>, and 3<i>d</i> levels of NaIV. The 3<i>s</i>, 3<i>p</i>, and 3<i>d</i> configurations show good <i>LS</i> purity.
© 1976 Optical Society of AmericaPDF Article